1. Field of the Invention
The present invention relates to an image distortion correcting apparatus for an image display device, and in particular to an image distortion correcting apparatus capable of correcting keystone distortion and pincushion distortion occurred in an image display device efficiently.
2. Description of the Conventional Art
Recently, among image display devices, a projection TV displaying enlarged TV images on a large screen has optical distortion inevitably. The optical distortion can be divided into pincushion distortion and keystone distortion. Herein, the pincushion distortion occurs by a CRT (cathode ray tube; CRT) and a lens, and the keystone distortion occurs by reflection through a reflector.
FIG. 1 illustrates optical distortion occurred in a projection TV.
As depicted in FIG. 1, the projection TV includes CRTs (respectively having a lens for enlarging a video signal) 11, 12, 13 for enlarging an input video signal and outputting it; a reflector 14 for reflecting an image respectively outputted from the CRTs 11, 12, 13; and a screen 15 for displaying a RGB image reflected onto the reflector 14.
Operation of the projection TV will be described.
First, the three R, G, B CRTs 11, 12, 13 are not arranged at the same position but rather are arranged side by side. Each R, G, B CRT 11, 12, 13 enlarges a received image and outputs it to the reflector 14. The reflector 14 displays the enlarged image on the screen 15. Accordingly, the screen 15 displays a certain enlarged color image.
However, because the image displayed on the screen 15 is affected by the plural CRTs 11, 12, 13 and the reflector 14, optical distortion occurs inevitably in displaying of the image. In more detail, in case of an image displayed on the screen 15 in the projection TV, according to relation between arranged position of the CRTs 11, 12, 13 and reflection position of the reflector 14, electron beam locus of the image respectively outputted from the CRTs 11, 12, 13 may not be coincided with each other. Accordingly, crooked or angled electron beam locus may occur on the center, edge, top/bottom and left/right of the screen 15. As described above, image distortion occurs inevitably in the projection TV, and accordingly quality of an image displayed on the screen is lowered. Herein, the electron beam locus means a straight line connecting each CRT 11, 12, 13 to an image-displayed point on the screen 15.
FIG. 2 illustrates simulation results of optical distortion occurred in a projection TV.
As depicted in FIG. 2, in order to show optical distortion occurred in the projection TV, simulations were performed by using a reference grid image 21. First, a first grid image 22 shows a keystone distortion occurred in the reference grid image, and a second grid image 23 shows a pincushion distortion occurred in the reference grid image 21. In addition, a third grid image 24 shows both a keystone distortion and a pincushion distortion occurred in the reference grid image.
As described-above, in the reference grid image 21 of the three CRTs 11, 12, 13, because each CRT has a different reflection point on the reflector 14 and has a different reflection path to the screen 15, the keystone-distorted image 22 having a ladder shape occurs. And, the pincushion-distorted image 23 symmetric on the top and bottom-right and left on the basis of the center of the screen 15 occurs. Herein, the farther from the center of the screen 15, the more severely distortion occurs.
Accordingly, in the reference grid image 21, because the three CRTs 11, 12, 13 are not installed at the same position but rather are arranged in parallel, a degree of the keystone and pincushion distortion is different at the top, bottom, right and left of an image, because the image can not be displayed at the same point of the screen 15, and accordingly a misconvergence phenomenon occurs. Because of that problem, picture quality of the projection TV is lowered.
In order to solve the above-mentioned problem, an image distortion correcting apparatus using a convergence yoke is widely used.
FIG. 3 is a schematic block diagram illustrating a concept of an image distortion correcting apparatus using a convergence yoke in accordance with the conventional art.
As depicted in FIG. 3, in the image distortion correcting apparatus using the convergence yoke, by installing a convergence yoke 32 at a CRT 31 receiving a video signal and applying an appropriate correcting current to the convergence yoke 32 in the horizontal and vertical directions, a path of a video signal outputted from the CRT 31 is adjusted. Accordingly, the optical distortion of a projection 33 can be corrected, and the convergence-adjusted video signal can be displayed on a screen.
However, the image distortion correcting apparatus using the convergence yoke shows nonlinear characteristics such as severe optical distortion, it is very intricate to correct the optical distortion and maintain a high resolution of the image continually after the convergence is adjusted. In addition, in the correcting apparatus using the convergence yoke, in order to get a current received to the convergence yoke 32, a control signal having non-linearity has to be generated, the received current has to be amplified, and accordingly complicated operation and devices are required to process the optical distortion.
In addition, because the correcting apparatus using the convergence yoke uses variation of a magnetic field by adjusting a current received to the convergence yoke 32, it is influenced by the earth magnetic field, and accordingly re-correction is required after performing the distortion correction. In that case, a product has to be re-called, it is not efficient for not only a user but also a manufacturer.
In the meantime, recently not only a CRT-based projection TV but also a LCD (liquid crystal display; LCD)-based projection TV has been provided. In comparison with the CRT, because the LCD is thinner and lighter, it is easy to move, and accordingly a wall-type TV impossible to implement with the CRT can be implemented with the LCD. In addition, the LCD has smaller power consumption, it is efficient in the economical aspect. However, the image distortion correcting apparatus using the convergence yoke can not be applied to the LCD-based projection TV, a new image distortion correcting method has been required.